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This invention relates to a technique to evaluate current alarm settings for continuous and semi-continuous processes. The historical record of the alarm events and process measurements is used to determine alarm settings that balance providing adequate warning with minimizing false alarming during normal operation.
Semi-continuous processes are defined as processes that have discrete states, but the states may be transformed into continuous form. For instance, a measurement of discrete events is reformulated as discrete events per minute.
Alarms are often applied to process measurements to indicate that an event is likely to occur in the near future. There is a trade-off in determining the actual alarm setting. If it is set too near to the actual event, it will not provide adequate warning of the event. If it is set too close to normal operation, it will indicate an incipient event condition even though the process is still within a safe range. An event is defined as an undesirable situation such as an outage of service or a hazardous operating condition, or any other condition that requires attention.
The actual value of the alarm is generally set in an informal way- either by a designer or by an operator. The designer knows the normal operation value and the event value, and, based often on heuristics, sets the alarm value somewhere in between. The designer does not in general know two critical pieces of information: the normal process variation, and the speed which the process can be brought back to safe conditions once an alarm has occurred. Operators may be more familiar with these values, but their knowledge may be limited; most alarms only occur occasionally, and they are usually not examining whether the alarm setting is optimal when an alarm occurs.
Standard Statistical Quality Control (SQC) techniques can be used to ascertain whether a process is in an alarm condition (see for example U.S. Pat. No. 5,257,206), but they are typically binary indicators (i.e., indicating either that the process is in or out of alarm), and are not designed to indicate the likelihood that the process is going to alarm in the near future. Furthermore, these techniques were developed for discrete parts, and several assumptions underlying their formulation are not strictly valid for continuous processes (MacGregor, J.F., On-Line Statistical Process Control, Chem. Eng. Prog., 84(10), 21-31, 1988). In addition, SQC techniques do not provide an indication of the continuance of the system after an alarm has occurred (as is considered in this patent application).
Another technique is to differentiate between normal and abnormal conditions is to use a deterministic model to compare the model predictions to actual conditions. For example, in U.S. Pat. No. 5,493,729, an expert system is employed to infer causal relationships between events. In U.S. Pat. No. 5,997,167, a deterministic model is coupled with normally-distributed model parameters to indicate the most likely state of the system, and then take appropriate action. The techniques of this application, in contrast, do not require a deterministic model as a statistical model (which will most likely not be based on a normal distribution) is constructed solely from past histories of the plant. Further, it is not an objective of this invention to model the behavior of the process or to determine whether a process should be in an alarm state, but more strictly to determine whether the current alarm setting is acceptable, and whether future alarms can be predicted.
It is a feature of the present invention to provide a method to determine the most desirable alarm settings for a continuous or semi-continuous process, and to evaluate the effectiveness of the current settings. It is, also, a feature of the present invention that it only requires normal operating data to determine the most desirable settings and evaluate the current settings. Yet another feature of the present invention is that the identified alarm setting adequately reflects the objectives of providing adequate operator warning and minimizing false alarming.
Additional features and advantages of the invention will be set forth in part in the description that follows, and will in part be apparent from the description, or may be learned from practice of the invention. The features and advantages of the invention may be realized by means of the combinations and steps pointed out in the appended claims.
Accordingly, objects and advantages of the present invention are:
a) to determine desirable alarm settings for continuous and semi-continuous processes using only normal operating data;
b) to evaluate the effectiveness of the current settings;
c) to indicate the ramifications of changing the alarm setting over a range of values.